The invention relates generally to optical fiber measurement and more particularly to a method for dispersion measurement of uncut fibers.
Optical fiber has become increasingly important in many applications involving the transmission of light. When light is transmitted through an optical fiber, the energy follows a number of paths that are called modes. A mode is a spatially invariant electric field distribution along the length of the fiber. The fundamental mode, also known as the LP01 mode, is the mode in which light passes substantially along the fiber axis. Modes other than the LP01 mode, are known as high order modes. Fibers that have been designed to support only one mode with minimal loss, the LP01 mode, are known as single mode fibers. A multi-mode fiber is a fiber whose design supports multiple modes, and typically supports over 100 modes. A few-mode fiber is a fiber designed to support only a very limited number of modes. For the purpose of this patent, we will define a few mode fiber as a fiber supporting no more than 20 modes at the operating wavelength band. Fibers may carry different numbers of modes at different wavelengths, however in telecommunications the typical wavelengths are near 1310 nm and 1550 nm.
As light traverses the optical fiber, different groups of wavelengths travel at different speeds depending on their wavelength, which leads to chromatic dispersion. Chromatic dispersion is defined as the differential of the group velocity in relation to the wavelength in units of picosecond/nanometer (ps/nm). In optical fibers the dispersion experienced by each wavelength of light is also different, and is primarily controlled by a combination of the material dispersion, and the dispersion created by the actual profile of the waveguide, known as waveguide dispersion. Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion in this patent refers to chromatic dispersion. The units of total dispersion are in ps/nm, and a waveguide fiber may be characterized by the amount of dispersion per unit length, in units of ps/nm/km.
The differential of the dispersion in relation to wavelength is known as the slope, or second order dispersion, and is expressed in units of ps/nm2. Optical fibers may be further characterized by their slope per unit length of 1 kilometer, which is expressed in units of picosecond/nanometer2/kilometer (ps/nm2/km).
Measurement of chromatic dispersion of fibers in the fundamental mode is often accomplished by measuring the flight time differential of pulses of light at different wavelengths. In order to accomplish the measurement, prior art methods typically utilize a cut piece of fiber so as to measure the flight time of the pulse from the source of light to the end of the fiber whose dispersion is being measured. Unfortunately, cutting the fiber has negative consequences in that the cut fiber cannot be used if it is found to be too short for the desired application. It is possible to splice different pieces of fiber together, however this is not always desirable, as additional losses are incurred as a consequence of the splicing operation.
Few mode fibers designed to have specific characteristics in a mode other than the fundamental mode are also known as high order mode (HOM) fibers. The operative high order mode is also known as the desired mode. HOM fibers are particularly useful for compensating chromatic dispersion due to the large amount of negative dispersion that can be experienced by a signal traversing certain profiles in a high order mode. Unfortunately, HOM fibers are much more sensitive than single mode fibers to the actual profile of the fiber, and therefore it is desirable to characterize manufactured fibers in order to determine their actual dispersion characteristic prior to having the fiber cut to length. For HOM fibers, it is necessary to launch the light in a specific high order mode, which requires the use of a mode transformer. Splicing is disadvantageous, in that in addition to losses inherent in a splice, additional undesired modes are typically created. Prior art methods of measurement involve cutting the fiber to an estimated maximum length, measuring the actual fiber, and then proceeding to cut back further lengths and remeasure until the desired dispersion is achieved. These cut backs reduce the amount of fiber that is eventually useable, and thus leads to a large amount of waste.
Thus there is a need for a method of dispersion measurement of optical fibers that does not require cutting the fiber to a useable length prior to measurement.
Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art methods of dispersion measurement. This is provided in the present invention by an apparatus for measuring dispersion of a section of optical fiber beginning at one end of the fiber and ending at a point before the second end of the fiber comprising a light source that generates an optical measurement signal, the light source being optically connected to one end of the optical fiber whose dispersion is to be measured, a loop designed to modify the propagation of the optical measurement signal that is formed in the optical fiber at a point to which dispersion is to be measured, and a time delay measurement apparatus, wherein the wavelength of the optical measurement signal is variable and the time delay measurement apparatus is operable to detect the change in propagation time from the one end of the optical fiber to the loop as a function of wavelength.
In one embodiment the apparatus further comprises a pulse generator being operable to modulate the light source. In another embodiment the apparatus further comprises a detector operable to detect the modification to the propagation of the measurement signal.
In an exemplary embodiment the optical fiber is a high order mode fiber. In a further exemplary embodiment the apparatus comprises a mode transformer operable to transform the optical measurement signal to a high order mode.
In one embodiment the modification to the optical measurement signal comprises increasing the amount of detectable light escaping from the optical fiber, while in another embodiment the modification comprises increasing the amount of loss experienced by the optical measurement signal.
In one embodiment the light source comprises a tunable laser. In one exemplary embodiment the time delay measurement apparatus comprises an oscilloscope and in another exemplary embodiment the time delay measurement apparatus comprises an optical time domain reflectometer.
The invention also provides for a method for measuring dispersion of a section of optical fiber beginning at one end of the fiber and ending at a point before the second end of the fiber comprising the steps of generating an optical measurement signal, propagating the optical measurement signal in an optical fiber, modifying the propagation of the optical measurement signal at the desired measurement point, varying the wavelength of the optical measurement signal and measuring the change in propagation time from the one end to the desired measurement point as a function of wavelength.
In one embodiment the optical measurement signal comprises optical pulses. In another embodiment the method further comprises detecting the modification to the propagation of the measurement signal.
In an exemplary embodiment the optical fiber is a high order mode fiber. In a further exemplary embodiment the method comprises the step of transforming the optical measurement signal to a high order mode.
In one embodiment the modification to the optical measurement signal comprises increasing the amount of detectable light escaping from the optical fiber, while in another embodiment the modification comprises increasing the amount of loss experienced by the optical measurement signal.
In one embodiment the optical measurement signal is generated by a tunable laser. In one exemplary embodiment the measuring is accomplished by an oscilloscope and in another exemplary embodiment measurement is accomplished by an optical time domain reflectometer.